1. Field of the Invention
The present invention relates to a plasma display device employed in a flat-panel television, an information displaying device, and the like, and more particularly to a plasma display device which suppresses image sticking caused by light emission at high luminance.
2. Description of the Related Art
In case of a display device such as a plasma display device, displaying an image over a long period causes image sticking due to deterioration of phosphor or the like. In particular, the plasma display device performs displaying by means of photoluminescence of the phosphor that uses vacuum UV rays generated by the discharge. For this reason, deterioration occurs on the surface of the electrode by the discharge, and in the phosphor by ion bombardment and exposure to vacuum UV rays. The deterioration on the surface of the electrode and in the phosphor depends on the number of discharges. In other words, the higher the luminance, that is, the greater the number of discharges per unit time, the faster the phosphor deteriorates, and illuminating the phosphor partially at high luminance causes a difference in luminance from the surrounding portion, which causes image sticking to occur in a short time.
FIG. 1 is a block diagram showing a major portion of a conventional plasma display device. FIG. 2 is a graph showing a relation of load of display versus luminance in the conventional plasma display device.
As shown in FIG. 1, the conventional plasma display device is provided with a control circuit 12, which controls luminance on a plasma display panel (PDP) 11 in response to load (a quantity of load) read on an input signal. The control circuit 12 controls the luminance in a manner as shown in FIG. 2. More specifically, the control circuit 12 lowers the luminance as the load of display increases, and heightens the luminance as the load of display decreases.
The conventional plasma display device adopts such a control method that it can display a sharp motion picture. Consequently, in case of a scene having a partial highlight region, such as a scene of the sunrise or metal reflecting sunlight, the luminance in the partial highlight region is heightened further to be emphasized, so that such a scene can be displayed sharply. On the other hand, there can be offered an effect that power consumption is saved when displaying a scene that glows across the screen.
However, when the foregoing conventional plasma display device is chiefly used to display fixed characters like a display board, as has been discussed, luminance is high and therefore the number of discharges increases at a partial highlight region, for example, a character portion displayed in white on the black background. Thus, luminance-induced deterioration occurs at different speeds in the white character portion and the black background portion, in other words, the white character portion deteriorates faster than the black background portion. Hence, when a different image, in particular, a monochrome image in white is displayed on the entire screen, a problem occurs that the portion used in displaying the white characters becomes darker than the rest, which is often recognized as image sticking. In case that the display area is small and load is low, in particular, the luminance at the display portion reaches its maximum, which accelerates the deterioration in the phosphor, thereby raising a problem that the image sticking occurs in a short time. In an extreme case, image sticking is observed within several tens of hours when a fixed pattern is kept displayed in a small display area.
On the other hand, if the luminance is limited in order to suppress the occurrence of the image sticking and extend the service life thereof, the luminance and contrast decrease in all the display patterns over the entire display period. This causes a problem that both the display quality and visibility deteriorate.
It is an object of the present invention to provide a plasma display device capable of suppressing the occurrence of image sticking while maintaining a high display quality and satisfactory visibility.
A plasma display device according to the present invention comprises a plasma display panel and a luminance controller. The luminance controller lowers a luminance on the plasma display panel when there is no change in an input signal for a predetermined position in the plasma display panel within a predetermined time than a luminance when there is a change in the input signal within the predetermined time.
In the present invention, because the luminance controller controls the luminance in relation to whether there is a change in an input signal within a predetermined time, adequate luminance can be set separately when displaying a still image, which neither has a change in an input signal nor requires high luminance, and when displaying dynamic images, which require high luminance. Accordingly, it is possible to maintain a sharp image quality when displaying dynamic images, while suppressing the occurrence of image sticking by controlling the luminance when displaying a still image.
Also, by setting the luminance to a constant level when there is no change in the input signal regardless of the load, the image sticking on the plasma display panel can be suppressed in a reliable degree. If it is designed so as to lower the luminance as the load increases when there is no change in the input signal, a change in luminance between when there is a change in the input signal and when there is no change in the input signal becomes smaller, thereby improving the visibility compared with when the luminance is set to a constant level. Further, if a plurality of luminance levels when there is no change in the input signal are provided relating with time lengths during which there is no change in the input signal, the luminance may be further lowered when there is no change in the input signal over a longer period of time, thereby making it possible to suppress the occurrence of the image sticking further.
Also, by locating the predetermined position at one of four corners of the plasma display panel, the device may be suitably used in chiefly displaying fixed characters, such as diagrams which change the screen almost entirely, charts, and text information.
Further, in case that the input signal is a signal in which a predetermined format varies, the predetermined position may be set to a position determined arbitrary in the plasma display panel. Accordingly, the device is suitably used as an information board in public facilities, timetables for train, airlines and the like.